non-marine sedimentary environments

Clastic Depositional Environments

The main environments for coarse clastic deposition

• Fluvial Environments• Deserts• Lacustrine Environment• Deltas• Shoreline Marine Environments• Shallow marine shelves and epeiric seas• Continental margins and deep water basins• Glacial Environments

Fluvial Sedimentary Environments

Alluvial Fans

The Start of the Sedimentary Cycle

• Bedrock weathered away from uplifted areas (mountain ranges)

• Carried away in mountain streams• Start the process of building up

sedimentary deposits.• First of these deposits to form: Alluvial

fans

How do alluvial fans form?

• When a narrow (confined) canyon stream disgorges onto a valley floor

• Sudden deceleration in flow and in gradient– Decreased ability in the stream to carry

coarser material: this is dropped.• Results in a cone-shaped deposit of

coarse stream sediments, sheet flood deposits and debris flows: Alluvial Fan

Typical structure of an alluvial fan

RADIAL FAN SECTION

FAN SURFACE

RADIAL PROFILE

• Alluvial fans best known from arid/ semi-arid environments, where periodic flow occurs in the canyons but also occur in humid environments.– Usually triangular in map view and wedge-shaped in

cross section.– Slopes range from 1 – 25°, average 5-10°.– The larger the particle size, the steeper the slope.

• Described as “active” when the fan is building or “inactive” when it is not.– To be active: must be continued uplift and erosion of

highlands to supply sediment: fault scarps are common sites of alluvial fans.

Typical structure

of an alluvial

fan

RADIAL FAN SECTION

FAN SURFACE

RADIAL PROFILE

• Alluvial fans can build out onto:– Playas– Lakes– Floodplains of permanent rivers– Coastal plains– Directly into the sea (fan deltas)

• The surface of the fan is dissected by radiating channels

What sort of sediments are found on alluvial fans?

• Generally coarser than other fluvial deposits (short transport distance)

• Compositionally immature• Grain size normally decreases down-fan• Upper- Mid- and Lower-Fan facies can be

distinguished.

Transport of material on alluvial fans

• Three methods:– Debris flow– Stream flow– Mud flow

Debris Flow on Alluvial Fans• When sediment becomes saturated with water: flows as

a viscous plastic mass: behaves like quicksand• High density – High Viscosity Flow• Debris flow can carry very large boulders and also clays

and fine particles• Results in very poorly sorted deposits with little or no

stratification• Sometimes the base of a debris flow shows inverse

grading (grain size increases upwards)• Generally form lobes in the upper reaches of the fan.

Debris Flow on Alluvial Fans

• Produces laterally extensive beds without erosive bases

• Matrix-support fabric

Stream Flow/ Stream Floods• In arid environments• Flash floods in canyons: extreme erosional power.• Low viscosity Flow• As flow velocity decreases, bounders, cobbles and

pebbles are dropped (clast supported fabric).• Results in a flow that is choked with more sediment than

it can carry: braided streams form on the fan (dry up quickly)

• Each flood cuts new channels, filling old ones with gravel.

• Produce cross-bedded pebbly sandstones with imbrication: generally upper / mid-fan deposits

Stream Flow: Sheetflood deposits

• At high flood levels, sand and gravel-rich flow covers the mid-fan (N.B. no fine material): Sheetflood Deposits

• Typically well sorted, stratified and cross-bedded.• Commonly form lobes than emerge from the channel at

the intersection point of the fan surface & the channel profile

• Little silt/ clay so water flows freely through these deposits without blocking the pores so these lobe deposits are commonly called sieve deposits

• These become progressively coarser towards the front of the lobe, where gravel accumulates.

• Sieve deposits are normally proximal/ upper mid-fan deposits.

Schematic profile of a sieve lobe

deposit

Mud Flow deposits on alluvial fans

• Where the debris flow is primarily fine particles

• Forms restricted narrow lobes like debris flows

• Mudflows that are more fluid can form enormous sheetflood deposits (>10km/h)

• Very fast moving, very dangerous deposits.

Typical depositional structure in alluvial fans

• Require rapid uplift: commonly found in– Rapidly downdropping grabens– Foreland basins– Strike-slip basins

• Typical profile:– Mixture of unsorted debris flows– Stream channel conglomerates (fanglomerates)– Cross-bedded sandstones– Sieve deposits– Commonly coarsen up in the stratigraphic record

• Sequence:– Cross-bedded ssts of distal fan at base– Overlain by coarser proximal fan deposits as uplift continues– Thin fining up sequence of fan decay on top

Can be very thick: 9000m of fanglomeratesat margin of San Andreas Fault.

Fluvial Sedimentary Environments

Braided River Systems

Basic features

• High discharge of water/ high sediment load / high topographic gradient

• Network of low sinuosity channels• Often originate on alluvial fans: rapid erosion and

little vegetation to stop run-off• Rapid erosion

– channels quickly become choked with sediment– Almost as soon as channel is scoured, it is infilled– So much sediment can fill the channel that it pokes out

of the surface: gravel bar (may become vegetated)– Rapid divergence of channels due to gravel bars

Braided river channels• Generally broad and shallow channels• Where sand grade dominates: commonly floored

by dunes• Sand bars and large dunes (straight crested)

divide the stream into smaller channels• Sand bars and dunes are exposed at low

discharge levels• Can have smaller ripples and dune developed

on them at this stage• The dunes and bars migrate downstream and

alter the positions of the channels

• In coarse grained braided systems:• Commonly have flat bedded imbricated

gravel channel deposits– These usually have thin, rippled and dune

sandstone tops (bar top sandstones)• Fine grained deposits are poorly

developed (only in abandoned channels)• Braided systems usually aggrade, but can

migrate laterally

Dominant components of braided facies

• Normally dominated by channel and bar facies with tabular, planar bedding (downstream migration of bars and dunes)

• Very little fine grained sediment• No overall fining up motif• Some trough cross bedding may be present• Internal erosion surfaces are common• Channels may show some fining upwards• Braided systems generate

– Elongate multistorey sand bodies with sheet geometry (depending on lateral migration)

– Mudrocks are missing or rare– Palaeocurrents are unidirectional with low dispersion

Fluvial Sedimentary Environments

Meandering Stream Facies

Meandering streams• Confined flow: Possess distinct channel &

overbank subenvironments• High sinuosity meandering streams develop in

regions of low discharge and low topographic gradient.

• Associated vegetation inhibits widespread erosion and limits sediment supply.

• Produced by confined flow with periodic overflow of banks

• The typical succession and general pattern of sedimentation is shown in the following diagram:

• The Channel– Formed by lateral movement erosion and deposition– generally has large dune structures on its floor– Often has coarse channel lag deposit– Generates a fining up motif with point bar above lag– Often contains mud pellets from river bank

• The Point Bar– Dunes also occur on lower part of point bars– These produce trough cross beds (compare with braided)– Flat bedded sands of upper flow regime may also occur on the

point bar – in the uppermost part, rippled sands produce cross laminations

and mud drapes

Gravel deposited in a channel

Graded Bedding in Fluvial Deposit

• Alluvial Ridges– Produced by migration: erosion and deposition of point bar

produces a region higher than the rest of the alluvial plain.• Levees

– are built up from fine sands and silts that are deposited duringriver high stages

• Crevasse channels– can be cut through the channel bank to bring coarser sands onto

the floodplain in crevasse splays– This produces thin sandstone/ siltstone interbedded deposits

• Overbank deposits– Overbank flooding carries suspended fine material onto the

floodplain– Often evidence of drying out– Floodplains can also be sites of soil formation, marshes,

swamps, lakes…

Mudcracks formed in overbank

• Meandering streams may abandon channels during their development.– Form Oxbow lakes gradually filled with fine

grained sediment (clay plugs)– These generally help to keep the river to its

alluvial ridge but occasionally the river breaches these to build a new alluvial ridge on the floodplain

• This process is known as avulsion

Meandering Stream Sedimentation Patterns

• Generates a fining upwards succession– through point bar migration

• Occasional floods smooth off the point bar to form epsilon cross bedding of the lateral accretion surface– Always dips normal to palaeoflow direction

• Thick floodplain deposits with channel point bar sandstones characterize the meandering environment

• Point bars may be connected to each other or may be separated by floodplain silts.

Fining up in a fluvial

system

Comparison: Meandering and Braided Rivers

BedloadSuspendedLoad transport

Sand and gravelMud and siltPredominant sediment load

Sand and gravel at margins

Mud & Silt mainly at margins

Channel cross section

Shallow but WideDeep but NarrowSediment cross section

LOWHIGHChannel sinuosity

YESNOAlluvial Island Bars formed

Desert Sedimentary Environments

Aeolian sands, playas & ephemeral lakes/ streams

• Arid deserts: 20° - 30° latitude in areas of constant high pressure or rain shadow (mountains)

• Wind less efficient: coarser material left as deflation lag or desert pavement, sand/ finer material transported.

• Desertic environments are NOT just sand dunes: a series of subenvironments exist.

• Desert character depends upon balance between – wind strength (can exceed 190kph), – rate of sediment supply– effectiveness of vegetation binding.

• Arid climate: particles are weathered in upland regions and move to lowland regions in ephemeral streams, which pass through wadis.

• Aeolian deposits will be developed in a relatively arid setting where there is more sand being transported into the system than is leaving it.

• Particles deposited in lowlands coalesce into sand seas or ergs.

• Ripples and dunes build up and migrate across the ergs (and the ergs themselves move) unless the sand is fixed by vegetation (savannah).

• Sand systems will eventually reach the sea (longest route=5000km).

• Particles move long distances, hence sphericity and roundness.

• Remember: sand can move uphill, unlike water.

High winds & Low Sediment Supply

• Rock & Gravel Pavements– Finer grains (clay and silt grades) are transported in

suspension as red dust storms, sometimes for 1000’s km before they settle from the air

• Where supply of sediment does not match the rate of erosion, – the sand may be removed down to the damp water-

table. – This deflation can give a very distinctive planar

erosion surface in aeolian deposits.– Sand transport will resume when more sediment is

supplied or the water-table drops still further.

Bedforms common in deserts

• Dunes, ripples…• Cross-bedding (very large scale)• Evaporites common in dried out Playa

lakes (interdune)• Reddening of sediments common.• Most importantly: aeolian sands are

marked by definite truncation surfaces

Saltating Sand

Big Dunes

• No limit on the overlying fluid so the dune size is limited only by wind strength.

• Each cross set can be up to 35m thick with foresets dipping 20-35° (steeper than water lain deposits)

• Cross beds are generally long sweeping features with asymptotic bases

Barchan Dunes

Aeolian Cross-bedding

Dune structures

• Low amplitude wind ripples can migrate up the lee faces of aeolian dunes: not found in fluvial systems.

Modern Wind Ripples

Ephemeral Streams & Lakes

• Between dunes• Thin mudstones with rain pits and

dessication features: ephemeral lakes• No regulr progression between dunes and

interdune deposits.• Ephemeral lake formation often a random

process

• Low, flat "interdune" areas lie between the crescentic dunes.

• Playa Lakes: commonly fine grained lacustrine style deposits in the interduneareas. Often dry out: evaporitic.

Recognizing aeolian systems

• Roundness, sphericity• High-angle cross beds• Very thick cross bed units• Reddened• Interdune deposits of muds and evaporites• Very texturally and mineralogically mature

sediments

Fossils in the desert

• Commonly reptile/ dinosaur trackways.

Shoreline Deposition

What governs the depositional style?

• Rate of water discharge from rivers• Rate of sediment influx from rivers• Tidal regime and action of waves• Larger scale marine currents• Climate and vegetation• Geometry of shelf slope/ marine basin• Relative movements of land and sea, rate of

marine basin subsidence

• Modern shorelines: some erosional/ some depositional.

Depositional Shorelines• Interplay between sediment supply from land

and ability of marine processes to remove it.• Produces a range of shoreline types from

sediment dominated to marine process dominated.

• Consequence of this interplay: Shoreline Shape• Low weight shorelines: Deltas built up• Linear Shorelines: Barrier Islands and lagoons.• Essential to distinguish between these in the

record.

River Mouth Flow Behaviour

• Water + Sediment in river enters the sea or lake: 3 possibilities

• 1. Inflow more dense: turbidity current deposits submarine fan

• 2. Equal densities: sediment dispersed radially into a narrow zone: gilbert delta

• 3. Inflow less dense: Build up of sediment due to deceleration: Deltas Build